1. Field of the Invention
The present invention relates a driver apparatus for driving a plurality of light emitting elements, a print head that incorporates the driver apparatus, and an image forming apparatus that employs the print head.
2. Description of the Related Art
Some existing image forming apparatus such as an electrophotographic printer employ an exposing unit in which a plurality of light emitting thyristors are aligned. A single driver circuit drives a group of light emitting thyristors. A drive signal is applied to the gate of a light emitting thyristor that should emit light, thereby causing current to flow from anode to cathode to emit light.
One known print head using light emitting thyristors is a self-scanning light emitting print head. For example, a power supply voltage of 3.3 V is not enough to trigger the gate of the light emitting thyristors. Therefore, a transfer clock signal is passed through a circuit that causes an undershoot voltage on the waveform of the transfer clock signal. Then, the power supply voltage of 3.3 V is added to the undershoot voltage to provide a signal capable of triggering the gate.
For example, Japanese Patent Application Publication No. 2004-195796 discloses the following technique. A transfer clock is outputted from a first output terminal of a clock driver, and is passed through a CR differentiation circuit to generate a waveform with undershoot. Then, the undershoot waveform is added to the output from a second output terminal in order to add a direct current component. The second output terminal is necessary to provide a direct current component since the CR differentiation circuit fails to provide a direct current component.
The above-mentioned conventional self-scanning light emitting print head requires a clock driver having two output terminals and therefore suffers from the following drawbacks.
In order to implement high speed operation, a print head incorporate a plurality of chips of self-scanning thyristor array that operate simultaneously in parallel. The data transfer clock for the arrays is a two-phase clock that feeds two clock pulses to each array chip. This implies that a clock driver for the self-scanning print head requires four output terminals for driving a single array chip.
The print head incorporates multiple self-scanning thyristor array chips, and therefore the clock driver must have a large number of output terminals. This leads to a large scale integrated circuit having a large number of terminals. However, a large scale integrated circuit is limited in the number of terminals that can be formed therein. If only a limited number of terminals are to be accommodated in a large scale integrated circuit, then the clock driver must drive a large number of chips in parallel. This heavy load causes the waveform of the clock to be distorted. As a result, the print head cannot operate at high speed.
The same is true for a self-scanning print head that uses light emitting diodes.
There is a need for an LSI circuit configuration of self-scanning light emitting array chips that can be driven by clock signals through as small a number of terminals as possible.